Wire cut electric discharge machining devices generally machine a thread-like slit corresponding to a fixed path in a workpiece, by means of generating an electric discharge between a wire electrode and a workpiece while causing a traveling wire electrode to move relative to a workpiece along a fixed path. Wire electrodes having a diameter of 0.3 to 0.01 mm are normally conveyed to a wire pulling device from a wire bobbin via multiple pulleys and a pair of wire guides which impart tension to the wire. During machining, the wire electrode travels between the pair of wire guides in a vertically stretched state. The path of movement of the wire electrode relative to the workpiece is described within a vertical plane (X-Y plane) in the direction traveled by the wire electrode. The operator normally creates the NC program which determines this path--a collection of program blocks--prior to machining. Such wire cut electric discharge devices have an extremely high machining accuracy and are suited to precision machining. The wire electrode may break in the midst of the movement path and machining due to the condition of small pieces of the workpiece produced by machining. Such undesirable wire breakage is detected, for example, by a sudden drop in the tension applied to the wire.
In general, when such a wire electrode wire break is discovered, the pair of wire guides is caused to move within the X-Y plane from the position where the machining stop to the start hole in the workpiece which is the starting point of the wire movement path. This movement is automatically executed based on an NC program. The wire electrode is then pressed through the start hole and connected to a device which pulls the wire. The diameter of the start hole is greater than the width of the machining slit, so the wire electrode can be easily pushed through the start hole. After this action to restore the continuity of the wire electrode from the wire bobbin to the wire tensioning device--the so-called "wire connection"--the wire, again tensioned between the pair of wire guides, is returned to the position where the machining stop.
However, when the machining stop position is separated from the start hole, time is required to restore the wire electrode. The operator may move the pair of wire guides by eye from the machining stop position to a position at which he judges it will be easy to manually perform a "wire connection" using, for example, a remote control device. In most cases, a location in the machining slit having a simple shape is selected for this purpose. The operator manually pushes the wire electrode through the machining slit, connecting the wire to a tensioning device.
As the operator has moved the wire guides along the machining slit by eye, as illustrated in FIG. 5(B), its position may not precisely match that of the wire movement path. In such cases, a bend occurs in the wire between the pair of wire guides 14A and 14B. In FIG. 5(B), the positional offset between wire electrode 22 and slit 74 is exaggerated, but in order to remove this undesirable bend it is necessary to restore wire electrode 22 to the wire movement path. It may be difficult for the operator to restore wire electrode 22 to the wire movement path by eye through manipulation of a remote control device.